SELECTIVE ASSEMBLY OF LAMININ VARIANTS BY HUMAN CARCINOMA-CELLS

BACKGROUND: The laminins are heterotrimeric basement membrane glycopro teins. Eight subunits that can be assembled into laminins have been ch aracterized and are known as: A, B1, B2, S, M, K, B2t, B1k laminin cha ins. Although many neoplastic cells secrete laminins and some of them even assemble basement membranes, the pattern of production of various laminin subunits remains to be explored. EXPERIMENTAL DESIGN: The exp ression of laminin was examined in several human carcinoma cells using a panel of specific cDNA probes as well as polyclonal and chain speci fic monoclonal antibodies. For this purpose a human laminin S chain 2 kb cDNA was isolated and characterized and used together with existing probes for laminin chains. RESULTS: All carcinoma cell lines had a hi gh level of expression of three light chains (B1, S and B2) mRNA. In c ontrast, the heavy chains of laminin, A and M, were expressed in negli gible amounts as detected by Northern blotting and PCR. The only excep tion was the HU-1 lung adenocarcinoma cell line which expressed signif icant quantities of laminin M chain mRNA and lower levels of laminin A chain mRNA. The presence in the HU-1 cells of translated polypeptides was demonstrated by immunofluorescence staining. The cells contained both B1 and S chain laminin in the cell layer, but preferentially secr eted the B1 chain into the culture supernatant as shown by Western blo tting. The 300 to 400 kDa M chain immunoreactive band was found in lam inin secreted into the culture medium of HU-1 cells. Immunoprecipitati on of biosynthetically labeled proteins showed that the M chain was sy nthesized as a complex with B chains. Little or no A chain laminin was detected in the culture medium supernatant. HU-1 cells also synthesiz ed the newly described laminin variant, epiligrin which was secreted i nto the medium. Thus, the HU-1 cells secreted two laminin variants: M- B1-B12 laminin and epiligrin into the culture medium. Immunostaining o f HU-1 nude mice tumors showed that tumor basement membranes contained M, B1, and B2 laminin and epiligrin immunoreactivity but apparently n o S chain. CONCLUSIONS: All human carcinoma cell lines produced lamini n chains B1, B2 and S, but no or little A or M. The only exception was the lung carcinoma cell line HU-1. Human HU-1 carcinoma cells in cult ure synthesize several homologous laminin chains and regulate the proc ess of assembly, secretion and deposition of laminin variants into tum or basement membranes. These data indicate that the tumor cells vary a mong themselves with regards to laminin production and that some of th em, like HU-1 may produce essentially all laminin chains simultaneousl y.